1. Field of the Invention
The present invention relates to a low-voltage electromagnetic riveting apparatus and method, and more particularly to a method and apparatus for controlled and efficient low-voltage electromagnetic riveting.
2. Background Information
Riveting machines are well known and in wide use throughout the aerospace industry, as well as in other industries. Rivets provide the best known technique for fastening an aerodynamic skin to a frame to provide a strong, aerodynamically smooth surface. Rivets are also used in the interior structure of an aircraft, since they are the lightest and least expensive way of fastening structural components together.
One form of riveting uses a low voltage electromagnetic riveting (LVEMR) system 100, as shown in FIG. 1. The LVEMR system 100 provides a controlled amount of energy in a single pulse and is typically smaller and less cumbersome than a pneumatic or hydraulic system. Further, the LVEMR system has almost no mass so it only has nominal reactionary forces. The LVEMR system 100 shown in FIG. 1 incorporates two electromagnetic actuators, a first actuator 101 and a second actuator 112, which are positioned on opposite sides of first and second workpieces 114 and 115, respectively. The first and second work pieces 114 and 115 are sandwiched together and a hole has been drilled through them to accommodate a rivet 93. The first and second actuators 101 and 112 each include a body 116 in which is positioned a driver 118 and a coil 120. A rivet die 92 is coupled to the driver 118 and is forced against the rivet 93. Also, there may be a recoil mass 123 which is typically secured to a rear surface of the coil 120. Extending from the recoil mass 123 is an air cylinder rod 124, which extends out of the body 116 into a two-chamber air cylinder 126. Associated pressure relief valves and other control elements are shown diagramatically as block 128. The elements of block 128 are responsible for initially positioning the driver 118 and its rivet die 92 against a head of the rivet 93.
Power is supplied to the system 100 by means of a power supply 130. A DC output from the supply 130 is used to charge a bank of capacitors in circuit 132 to a selected voltage. The voltage selected is based on the force necessary to accomplish the desired riveting task. The circuit 132 includes an electronic switch positioned between the capacitors and the coil 120.
A trigger signal from a firing circuit 134 activates the electronic switch, dumping the charge of the capacitor bank in circuit 132 into the coil 120. A current pulse is induced into the coil 120 causing strong eddy currents in a copper plate 119 located at the base of the driver 118. This creates a very strong magnetic field that provides a repulsive force relative to the coil 120. The driver 118 is propelled forward with a large force causing the rivet die 92 to upset the head of the rivet 93. A more detailed discussion of low voltage electromagnetic riveting can be found in U.S. Pat. No. 4,862,043, which is incorporated herein by reference.
Once the LVEMR system 100 has upset the rivet 93, a fastened assembly 140 is created as shown in FIG. 1B. The assembly 140 includes a deformed rivet 146, having a head 142 and a tail 154. The hole drilled into the first and second workpieces 114 and 115 includes a countersink 148 drilled into the second workpiece 115 to receive the head 142 of the deformed rivet 146.
Unfortunately, the fastened assembly 140, when produced by the LVEMR system 100 described above, has significant gaps 150 between the head 142 of the deformed rivet 146 and the countersink 148. The gaps 150 are undesirable since they could lead to early corrosion of the deformed rivet 146, causing it to weaken and prematurely fail. Accordingly, for the foregoing reasons, there is a need in the art for a controlled low-voltage electromagnetic riveting apparatus and process that mitigates the gaps 150 between the rivet head 142 and the countersink 148.